1. Field of the Invention
This invention relates to a direct current (DC) arc furnace for performing arc-melting of scraps and Sub materials.
2. Description of the Related Art
In general, DC arc furnaces are superior to three-phase (alternate current (AC)) arc furnaces in the following points:
Having a single cathode electrode, they have a small electrode unit requirement;
Only small noise occurs at the time of melting;
Since arc vertically extends from the single cathode electrode to the bottom electrode, a uniform temperature distribution can be obtained, thereby preventing occurrence of a hot spot; and
Since induction loss is prevented, energy can be used efficiently.
Therefore, DC arc furnaces have recently been more popular than three-phase arc furnaces.
FIG. 1 shows a conventional DC arc furnace. As is shown in the figure, the DC arc furnace 1 has a bottom electrode provided at a bottom portion to be used as an anode, and a graphite electrode 9 provided at an upper portion to be used as a cathode. At the time of operation, scraps and sub materials are put into the furnace, and then DC arc is created between contact pins 7 provided in the bottom and the upper electrode 9, to apply arc heat energy to the scraps so as to melt them. The contact pins 7 are contained in an iron wall case filled with a stumping member 8, and only upper portions of the pins 7 are exposed to the interior of the furnace.
The contact pins 7 are connected to a current board 5, which has its lower surface portion connected to terminals 3 and 4. The terminal 3 is connected to a cable 2, and supplied with power through the cable 2. The power is distributed to each of the contact pins 7.
The terminal 4 is a hollow member, and has an inner passage 4a with an upper opening. The inner passage 4a communicates with the air supply source of an air conditioner. Further, the upper opening of the passage 4a faces the lower surface of a base plate 6 located above the passage 4, for blowing air onto base plates 5 and 6 and the contact pins 7 so as to forcibly cool them.
Air-cooling aims to protect the base plates 5 and 6 and the contact pins 7 from the heat conducted therethrough and the Joule heat created at the contact pins 7, and to maximize the heat conductivity of the furnace bottom, thereby reducing the maintenance cost of the bottom electrode.
Here, note that the amount of heat to be removed from the base plates is determined by a product of the temperature difference between a coolant and a cooled member, the contact area therebetween, and the heat transmission coefficient. The temperature difference is the difference between the allowable temperature of a cooled member and the temperature of a coolant, and hence does not greatly depend on the kind of the coolant. The contact area is hard to increase in light of limitations in structure. The heat transmission coefficient depends on the thermal conductivity and the flow rate of the coolant.
In the case of using air as a coolant, the upper limit of the heat transmission coefficient is actually about 100 kcal/m.sup.2.h.deg. The arrangement of the contact pins 7 and the passage of cooling air are important elements for keeping the air flow around each contact pin at high level.
As is shown in FIG. 2, in the conventional DC arc furnace, the contact pins 7 are arranged in a spiral manner about the air injection passage 4a. This arrangement restrains variations in the cross section of the cooled air passage, thereby minimizing the range of variations in air flow so as to increase the cooling effect of the base plates 5 and 6 and the contact pins 7.
Recently, there is a tendency of increasing the number of contact pins of the bottom electrode in order to increase the capacity of the furnace. A large number of contact pins can prevent current concentration, which may damage a particular portion of each contact pin or cause leakage of molten steel out of the furnace.
In the above-described conventional DC arc furnace, however, the layout of the contact pins is limited and hence the number of the pins cannot greatly be increased, since they must be arranged in consideration of the flow of cooled air, as is explained before. This is a significant drawback in increasing the furnace capacity.